Efficiency loss and harmful emissions remain a concern for many engine systems. For instance, stoichiometric spark ignited engines suffer several efficiency loss mechanisms in their most basic configuration that reduce their fuel economy as compared to compression ignited diesel engines. Losses common in engine systems such as increased in-cylinder heat losses, increased pumping losses at part load, and higher combustion efficiency losses all contribute to the overall efficiency loss. Processes such as dilution contribute to a diesel engine's efficiency advantage but is less feasible on a homogeneous combustion engine. In particular, as the gasoline combustion process is “leaned out” through air dilution, combustion stability issues and efficiency losses mount quickly, having limited air-fuel ratios to work with without extraordinary, and costly, measures. A diesel engine having a relatively concentrated inhomogeneous combustion process often stably operates at relative air-fuel ratios greater gasoline limit.
Along with efficiency standards, the engine industry is also concerned with engine emissions. Even at dilute conditions, gasoline combustion exhibit emissions that are higher than acceptable by governing agencies and company standards. Current options for controlling post combustion catalytic reduction of NOx emission in a “lean” running engine include using a selective catalytic reduction (SCR) system based upon ammonia as reductant, which requires a second on-board system to supply ammonia. A lean NOx trap may also be used for controlling post combustion catalytic reduction of NOx. However, the lean NOx trap requires periodic regeneration or desorption of NOx in a reducing environment for chemical reduction.